Abstract : Poly(lactic acid), PLA, is a biocompatible and biodegradable polymer that can be produced
from renewable resources. As a result, it has raised particular attention as a potential
replacement for petroleum-based polymers. It is an aliphatic polyester with properties such as
high modulus, high strength, and biocompatibility and is thus a promising material for various
applications such as implants, drug encapsulation, and packaging. In the wake of low glass
transition temperature, PLA has a low heat resistance and its application is limited to those not
associated with high temperatures. In addition, this polymer suffers from a low degree of
crystalinity. Increasing the crystallization rate in many processing operations, such as injection
molding, is required.
So far, many routes have been found to improve the crystallinity of PLA. These methods
include using nucleating agents, plasticizers, and combination of nucleating agents and
plasticizers together. PLA crystallization in the melt state results in two slightly different
crystalline forms known as α and α’forms. This thesis compares the self-nucleation ability of
these two crystal forms by self-nucleation. This is achieved by comparing crystallization
temperatures upon cooling for samples previously crystallized at various temperatures and
then re-heated to a temperature in the partial melting range for PLA. In the second step, we
study the effect of molecular weight of PLA on the nucleation efficiency of PLA crystalline
phases. This part of the investigation opens a new pathway to understand the role of PLA
crystalline phases on the optimal condition for its crystallization kinetics.
Polymer processing operations involve mixed shear and elongational flows and cause polymer
molecules to experience flow-induced crystallization during flow and subsequent
solidification. The mechanical properties of the final products are significantly dependent
upon the degree of crystallization and types of formed crystals. Therefore, optimization of any
polymer process requires a good understanding of how flow influences crystallization. The
type of flow can play a significant role in affecting crystallization. For example, elongational
flow causes molecules to orient and stretch in the direction of extension, as in the case of fiber
spinning and film blowing, helping the process of flow-induced crystallization. An extensive
body of literature exists on flow-induced crystallization of conventional thermoplastics.
Having said that, less attention has been paid to the effect of shear and elongational flow on
the PLA crystallization kinetics. As investigated in the final part of this thesis, the effect of
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molecular weight on the shear-induced crystallization of PLA is reported. For this, low,
medium and high molecular-weight PLAs were prepared from a high molecular weight one by
a hydrolysis reaction. Next, by means of a simple rotational rheometry, effect of the shear flow
was examined on the crystallization kinetics of these three PLAs.